PM vs SMC

[Dan G.]

My 73 was tagged in the rear end and I'm triing to obtain the replacement parts. So far I've found that replacement panels for the front clip are SMC and replacement panels for the rear clip are PM. Was 1973 a transition year and could have been originally assembled with both? or was the whole car SMC and if so what problems would I incounter bonding PM quarter panels to the original upper deck panel and bonding seams? Any info?

[Chuck S.]

Hmmmm...PM = "Press Molded"?

Press molded describes the manufacturing method of the panel, and not the material itself. The earlier cars were made out of polyester based fiberglass, usually referred to as FRP.

Actually, every year beginning in maybe 69 was a transition year from polyester fiberglass to Gennite, an SMC-like material. My 70 has door skins, and T-top skins that are of the same material as your front clip. More panels of the later material were added each year. If the material in question is light gray with black specks and streaks, it's Gennite; if it's dark charcoal gray or black, sometimes with fibers visible in the surface, it's polyester fiberglass.

If any panel you're replacing is the later, light gray material, you should use epoxy based bonding adhesive...the epoxy based adhesive will work for either material, but the older polyester bonding adhesive will not work for the later SMC type materials.

[Warren F.]

Re: PM vs SMC

Dan,

My observation on '70 thru '72 cars and also owning a '73 model as well, is that I believe the transition to the thicker, heavier SMC panels started showing up on '72 models.

[John W.]

Re: Hmmmm...PM = "Press Molded"?

Chuck, forgive my reponse to an old discussion. I have problems with e-mail. Maybe this will get to you via this method. I am finally ready to prep my '66 conv after 35 years. I last painted vettes professionally in early seventies. Back when there was body work followed by laquer primer followed by paint period. My car is no roach. The only filler is on the seams of a replaced top surround panel. After I re-do the old body filler and block sand the stripped (in 1980) body I plan to apply some kind of build up product and block sand, then an epoxy primer and block and finally a high quality black urethane (Sikkens or Glazurit). I am leaning toward single-stage. My question to you is that term "gel-coat" seems to mean different products to different people. If you agree with my plan of attack, can you recommend a specific "system" of compatable products by name. Someday I will learn how to "post" on this site.
Now if I get my password correct, you will get this. Thanks much!!

[Wayne W.]

Re: PM vs SMC

The 73 was a late transition period. 74 was the first with the complete outer panels in SMC. The rear deck was the last piece in the change over. I think all 73s had the old polyester rear upper panel, so you are likely to encounter that with a rear hit. On these models it is advisable to treat all panels as if they are SMC. Any material made for SMC will work on the old panels.

[Patrick H.]

Re: PM vs SMC

Dan,

On a relative basis, original rear clips are quite common. If you have the opportunity I would highly recommend finding an original rear clip from a 70-73 and using that on your car. I don't see the need to use reproduction fiberglass parts for the rear clip.

I'd recommend that for the front, too, (using the proper year, of course) but they are much harder to find.

Patrick

[Chuck S.]

Re: Hmmmm...PM = "Press Molded"?

For Corvettes, the gelcoat I've heard most about is Eckler's catalyzed, polyester based product that goes on rough and is very hard to get flat again. This product has a wax-like emulsion that floats to the surface to keep air away from the gelcoat as it cures. The wax covering the surface makes working the gelcoat that much more difficult. There is another gelcoat typically used for boats ( I believe it's called PVA, or polyvinyl alcohol) which is much easier to work. I'm sure there are other pros, cons, and facts about each product that I don't know.

Some painters won't refinish a Corvette unless it's gelcoated first...it's like cladding the fiberglass surface with a thin shell of resin that seals the trapped oils and solvents that can ruin a new paint job. I'm no expert on any of this, but from I've read here, gelcoat is really un-necessary if your fiberglass is clean, free of trapped oils and solvents, and has no fresh repairs or repro fiberglass panels.

As for single stage urethane systems and products, I can only tell you what I know about and plan to use on my own car...PPG acrylic urethane. I have painted one vehicle with this system training for the real deal, but I'm now convinced there's no way to make this system look like it was painted in Saint Louis, or even Bowling Green for that matter. A really good professional may be able to spray acrylic urethane in such a way as to duplicate the fine orange peel of lacquer, but even a gifted amateur will be unable to produce a decent surface without runs, nibs, and boogers that doesn't have to be color sanded and buffed. By the time you're done with that, an acrylic urethane finish will have outstanding gloss with zero orange peel. It may heresy to put such a paint job on a 65, but I don't want to be worrying about bird doo on my 70.

If your fiberglass is clean and free of fresh repairs, I would skip the gelcoat and start with one to two coats of PPG DP74LF epoxy primer...it's red oxide just like the original primer. After about an hour or so dry time, apply three to four coats of PPG K36 acrylic urethane primer-surfacer...gray just like factory sealer. This stuff goes on thick, and you'll need about 1.7-1.9mm nozzle for good HVLP spraying. In fact, the pot life is about 30 minutes, extendable to about an hour or so with reducer, so don't plan on mixing the whole batch...I finally resorted to mixing it one gun cup at a time. If you want even heavier film build, I understand PPG now has a K37...that should provide plenty of filler for blocking it straight if that's your objective.

Let the primer-surfacer cure overnight, and then start your blocking. The ideal situation is to sand off about two coats, and leave at least 1.7 mils dft. The primer-surfacer has to be thoroughly sanded everywhere...if you leave a litte unsanded on any edge, nothing is going to stick to it, not even epoxy primer...you'll be looking at edge chips in the color right down to the primer-surfacer. Wash the car thoroughly, and wipe and dry the sanded primer-surfacer down with alcohol or alcohol/water mixture...I don't use wax and grease remover anymore; too much chance for fisheye.

Then apply one sealer coat of reduced PPG epoxy primer (2:1:0.5)...I plan to use DP50LF since it's a medium gray just like the factory sealer for my white car. After the sealer drys an hour or so, I plan to apply two coats of PPG Concept single stage acrylic urethane, and then a third coat mixed 50/50 with PPG 2021 acrylic urethane clear.

[Chuck S.]

PS...Sand Scratches

For your initial prep of the fiberglass, sand scratches are virtually a non-issue when using PPG K36 primer-surfacer. You can go more coarse in sandpaper grit in order to get excellent adhesion.

I have had to make last minute repairs on missed dings, and failed to cover 36 grit scratches with filler...I noted the grinding scratches as I was applying the first coat of epoxy primer and figured I would see them again at the end. By the time I had sanded the K36, the grind marks were fully filled and covered...no glazing putty required.

Well known custom painter and founder of House of Kolor, Jon Koslowski, insists on "good tooth" and says he sands metal bodied cars with 80 grit paper before paint. I can't bring myself to put 80 grit scratches on fiberglass for general prep, but another professional painter has told me that 80 grit is the maximum grit scratches permissable on fiberglass repairs.

The previous vehicle I painted was an SUV with a fiberglass rear compartment cover. The fiberglass was just beginning to have some UV damage in the weathered areas, and I used 100 grit discs on a random orbital sander to strip the paint and remove any damaged material. After refinishing with the PPG system (K36), there is nary a sand scratch anywhere in that top... it looks like red glass.